i965/wm: use proper API buffer size for the surfaces.

i965/wm: use proper API buffer size for the surfaces.

Commit 5bb5eeea fixes a bug indicating that the surfaces should have the
API buffer size. Hovewer it picked the wrong value.

This patch adds a new variable, which takes into account
glBindBufferRange() values. This patch fixes the following CTS
regressions:

ES31-CTS.shader_storage_buffer_object.advanced-unsizedArrayLength-cs-std430-vec-bindrangeOffset
ES31-CTS.shader_storage_buffer_object.advanced-unsizedArrayLength-cs-std430-vec-bindrangeSize

Signed-off-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Reviewed-by: Marta Lofstedt <marta.lofstedt@intel.com>